US authorities say the pilot of a helicopter fighting fires in California has died after his aircraft crashed.
The pilot was the only person on board the water-dropping helicopter, which crashed near the city of Coalinga.
California Governor Gavin Newsom has declared a state of emergency as authorities battle hundreds of blazes.
Thousands of people have had to flee their homes in areas near San Francisco after several quick-moving wildfires swept into the region.
Most at risk is the city of Vacaville, home to about 100,000, which lies between San Francisco and Sacramento.
Officials went door-to-door overnight in a frantic effort to clear homes.
According to fire officials, 50 structures have burned down in Vacaville, with another 50 damaged by the fire.
"We are experiencing fires the likes of which we haven't seen in many, many years," Governor Newsom told reporters.
The blazes are thought to have been sparked by a heatwave combined with nearly 11,000 lightning strikes which have hit the state over the last three days.
Earlier this week, one of the highest ever temperatures on Earth was recorded in California's Death Valley. Across the US western states on Wednesday, nearly 45 million people were living in areas under some form of excessive heat warning or heat advisory.
Mr Newsom said 367 known fires have been recorded so far, but noted that "the prospect of that going up is very real".
"As those lightning strikes spark, as you have a lot of smoke, you have a difficult time determining total number of fires until certain things clear and we have the opportunity to go to more remote parts of the state," he added.
The group of fires in the San Francisco Bay Area was driven by winds throughout Tuesday night and has now grown to 46,000 uncontained acres.
The fire, which is larger than city of Washington DC, has injured four people and threatens about 2,000 buildings in Vacaville, authorities say.
Video on social media shows flames burning through people's front yards in California's famed wine-producing Napa and Sonoma counties.
Smoke also blanketed the city of San Francisco, about 60 miles (95km) south of Vacaville, on Wednesday morning.
Smoke is resting over San Francisco this morning. Fires are actively burning in various parts of the Bay Area. Things are moving quickly in #Vacaville— Jobina Fortson (@JobinaABC7) August 19, 2020
We’re LIVE now: https://t.co/mHxj8wa5LD pic.twitter.com/R2b5h9hRO7
Fires are also burning in the southern part of the state, where officials described "extreme fire conditions" amid the heatwave.
Meanwhile, the state's energy operator has pleaded with residents to use less electricity or otherwise face blackouts that would leave millions of people without power for hours at a time.
On Tuesday night more than 37,000 customers across California lost power, officials told CNN.
So far this year the US has experienced fewer wildfires than in 2019. According to Reuters news agency, around 1.4m fewer acres have burned this year, but the pace is expected to accelerate in the next month as Santa Ana winds reach the south and El Diablo winds strike in the north.